1. Field of the Invention
The present invention relates to an amplitude optical modulator using a two-electrode DFB laser structure. It is used in optical telecommunications.
2. Discussion of the Background
The amplitude modulation of an optical beam is conventionally brought about by means of a birefringent crystal (e.g. of lithium niobate), to which is applied a modulated voltage. Such a procedure suffers from the disadvantage of introducing significant optical losses and of requiring the use of high voltages.
Another method consists of using a frequency-modulated laser source and then converting the frequency modulation, normally known as frequency shift keying (FSK) into amplitude modulation, normally called amplitude shift keying (ASK). This method is e.g. described in the article by D. MARCUSE, entitled "Computer Simulation of FSK Laser Spectra and of FSK-to-ASK Conversion" published in Journal of Lightwave Technology, vol.8, no.7, July 1990, pp.1110-1122.
However, in this method there is still a slight frequency modulation. This leads to a deformation of the optical signal during its propagation in the optical connecting fiber. Thus, it is known that the chromatic dispersion of a conventional optical fiber (type G 652), which is approximately 15 to 17 ps/km/nm, deforms the optical wave train and limits the information rate. Therefore this method suffers from disadvantages.
More recently other devices known as semiconductor amplifiers have appeared. They are more particularly described in the article by B. FERNIER, P. GARABEDIANE, E. DEROUIN and F. LEBLOND entitled "Chirp-Free 2.5 Gbit/s Amplitude Modulation/Gating in Two-Electrode Semiconductor Optical Amplifiers" published in Proceedings of ECOC 92. However, there is a rate limitation to such devices.
The present invention aims at obviating these disadvantages. To this end, it recommends the use of a per se known device, which is a two-electrode distributed feedback or DFB-type structure.
FIG. 1 gives brief details of this structure. It comprises a lower confinement layer 12, an active layer 14, an upper confinement layer 16, two electrodes 18, 18' and two current sources 20, 20'.
Such multi-electrode DFB structures are already known in the field of optical telecommunications, where they serve as a tunable optical filter when polarized below the threshold. This type of filter can be used as a photodetector or as a discriminator for frequency-modulated beams. Such a structure and such applications are described in the article by M. J. CHAWKI, R. AUFFRET, E. LE COQUIL, P. POTTIER, L. BERTHOU, H. PACIULLO and J. LE BIHAN entitled "Two-Electrode DFB Laser Filter Used as a Wide Tunable Narrow-Band FM Receiver: Tuning Analysis, Characteristics and Experimental FSK-WDM System", published in Lightwave Technol., vol.10, no.10, October 1992, pp. 1388-1397, as well as in the article by M. J. CHAWKI, R. AUFFRET, and L. BERTHOU entitled "1.5 Bbit/s FSK Transmission System Using Two-Electrode DFB Laser as Tunable FSK Discriminator/Photodetector", published in Electron. Lett., July 1990, vol.26, no.15, pp.1146-1147.